Method of and apparatus for controlling vapor temperatures



March 13, 1-956 P H KOCH 2,737,931

METHOD OF AND AbPA RATUS FOR CONTROLLING VAPOR TEMPERATURES Filed June 9. 1950 4 Sheets-Sheet l FIG.1

INVENTOR Paul H Koch BY ATTORNEY March 13, 1956 P. H. KOCH 2,737,931

METHOD OF AND APPARATUS FOR CONTROLLING VAPOR TEMPERATURES Filed June 9, 1950 4 Sheets-Sheet 2 OOOOQOOOQOO/OOOOOOOOOOO FIG.2

INVENTOR ATTORNEY March 13, 1956 p, H, KO 2,737,931

METHOD OF AND-APPARATUS FOR CONTROLLING VAPOR TEMPERATURES Filed June 9, 1950 4 Sheets-Sheet I5 WITHOUT GAS RECIRCULATION WITH GAS RECIRCULATION INVENTOR Pau/ H jfoch BY ATTORNEY March 13, P. H. KOCH METHOD OF AND APPARATUS FOR CONTROLLING VAPOR TEMPERATURES Filed June 9. 1950 4 Sheets-Sheet 4 INVENTOR ATTORNEY such as a steam turbine,

METHOD OF AND APPARATUS FOR CONTROL- LING VAPOR TEMPERATURES PaulH. Koch, Bernardsville, N. J., assignor m The Babcock & Wilcox Company, Rockleigh, N. J., a corpu ration of New Jersey Application June 9, 1950, Serial No. 167,073

Claims. (Cl. 122-479) This invention relates in general to an improved method of and apparatus for vapor generation and superheating, and more particularly, to a method and apparatus of the character described in which the vapor generated is superheated in convectionheated surface and control of the superheat temperature is desired over a relatively wide generating load range.

A modern steam boiler, for example, includes a relatively high percentage of its steam generating surface in the form of tubes lining the furnace walls which receive substantially all of their heat by radiation fromthe high temperature combustionzone and gaseous products of combustion. Such boilers are designed with suflicient. steam generating surface in the furnace walls and in advanceof a convection type steam superheater to absorb a relatively large amount of the heat released in the furnace, so that the temperature of the combustion gases contacting with the superheater tubes at the designed maximum continuous operating load will permit the superheater tube metal temperature to be maintained within safe limits and to avoid slagging of the superheater tubes when a slag-forming fuel is in use. In such boilers, the fuel burners are constructed and located at points such that no flame impingement on the furnace walls will result and ample space for completion of combustion within the furnace will be provided. Where a slag-forming fuel is being burned and the incombustible ash constituents are to be removed from the furnace bottom as; dry ash, it is essential that the burners be located with suflicient vapor generating tubes between the main combustion zone and furnace bottom to absorb sufiicient heat by radiation to reduce the gas temperature in the space therebetween to produce a predetermined superheated steam temperature at a given load. With a water cooled furnace convection type superheater boiler designed for operation over a relatively wide load range, the maximum flame temperature will remain substantially the same, but the superheat temperature normally decreases with decreases in load, due to the lower gas temperature. entering the superheater and to a lesser degree the decrease in gas mass flow resulting from the decreased amounts of fuel and air supplied at the lower loads. The lower gas temperature is due to the fact that the radiant heat absorption by the furnace wall tubes is reduced at lower loads,.

but not in direct proportion to the change in load, as the amount of furnace heat absorbing surface remains constant and radiant heat absorption is proportional to the fourth power of the absolute temperature of the source J of radiation.

With alower temperature and reduced mass flow of the? gases contacting the superheater tubes, the final superheat temperature progressively falls with the load. Steam boilers are now operating with steam superheat temperatures as high as 1050" F. The higher this designed temperature, the more important it becomes from the standpoint of turbine efiiciency and maintenance that the steam superheat temperature be maintained substantially constant over the designed operating load range.

Ithaslong been recognized that in. steam boilers of the character described the gas mass flow could be advantageouslypincreased from the standpoint of superheat temperature maintenance as the boiler load drops by supplying additional gases to the furnace, either in the'form of a higher percentage of excess combustion air being supplied to the furnace or in the form of inert combustion gases recirculated to the furnace from a location in the gas path downstream of the superheater. In both cases it was contemplated that an intimate mixing of the added air or gases with the gases generated by the newly burned fuel would be effected in the furnace in or downstream of the main combustion zone. Even if such increased gas mass fiow in the furnace at the lower loads should result in a furnace exit average or mean gas temperature lower than the mean gas temperature so that the descending ash particles will solidify before and losing heat substantially entirely by radiation to the furnace wall tubes. Slightly lower gas temperatures and velocities have been found to exist in the corners of a rectangular furnace and in the bottom section of a hopper bottom furnace due to low velocity eddy current conditions in these locations. With other operating conditions the same, the extent of the main combustion zone in a pulverized fuel fired furnace will primarily depend upon which of the main types of fuel burners now in use is employed, i. e. either a short flame turbulent burner or a long flame non-turbulent burner. With a short flame burner, substantially all of the air required for combu stion is mixed with the fuel during or immediately after its introduction into the furnace, while with a long flame burner the combustion air is supplied in a manner as to gradually mix with the fuel stream along its flame path.

It has long been recognized that steam utilizing devices, operate most efliciently when receiving their steam supply at a constant or uniform pressure and temperature over the designed turbine operating load range. Steam boilers may be readily designed at the designed full load, a result which will depend upon the ratio of furnace heat absorbing area to furnace volume as well as the gas temperature level, the increase in gas mass flow over the superheater tubes would more than offset any such decrease in furnace exit gas temperature, with the net result that an increased convection heating of the superheater tubes can be attained and consequently a higher superheat temperature.

7 However the use for this purpose of a greater percentage of excess air than required to insure complete combustion in the furnace would have the disadvantage of resulting in a greater heat loss to the stack and an increase in fan power requirements. If introduced directly with thefuel stream, the amount so introduced would be limited by the effect on fuel ignition of a progressively leaner fuel-air mixture. The use of recirculated inert gases for this purpose in the manner heretofore proposed, would also have substantial operating disadvantages. If the inert recirculated gases should be mixed with the combustion air and/or fuel and so introduced into the furnace, the maximum flame temperature in the furnace would be lowered due to retardation of ignition and combustion of the entering fuel and the necessity of heating the added gas to flame temperature, and if too great a quantity should be recirculated in this manner, fuel ignition might be lost. The introduction of such additional excess air or recirculated gases through the burner ports would involve either a considerable increase in the velocity of the entering fuel-gas mixture or a modification of the burner ports if the same entering answer velocity conditions are .to bemaintained, both .of which results would be undesirable. The introduction of such recirculated gases at a location between the main combustion 'zone and the furnace 1 gasexit would not I affect thefuelcombustion' to the sameextent, but would have thedisadvantage of resulting in .corresponding'less mixingof the recirculated gases andhigh .temperature 'combustion gases before they reach'the superheater tubes. Such .gas: recirculation methods would also result in increased ifan power requirements and a lower overall thermal efficiency. Theloss in overall thermal etliciency ofrthe .boiler with gas recirculation would be less than withlthe use of more excess air than requiredlbecause of the greater heatloss to'theistack with the increased excess air. fBecauseofLthe limited range of operating loads in which these: controlmethods can be used and the inherent decrease tin :overall thermal efiiciency involved, "these methods ofisuperheat temperature control have not been usedcommercially to anysubstantialextent.

The present invention involves a method of and apparatus for controlling superheat temperatures by "the recirculation of combustion gases fromialocation'inithe gas:fiow path downstream of thesuperheater into a 'fiuid cooled-furnace of a vapor generating and superheating unit-.of the character described at a special location relative to the entering: streams of fuel and air andthefurnace gas exit, whereby the disadvantageous characteristics of prior gas recirculation schemes for this purpose are largely avoided and agreater degree of control'over a greateriload-range is attained. In its broader'sense, the inventioniinvolves the introduction of a variable'stream of recirculated inert gases into the furnace at such a location-or locations that a relatively thick layer or stratum ofgases at atemperature substantially lowerthan that of. the gaseous products of combustion being generated is interposed between the main combustion zone and a substantial area of furnace Wall steam generating tubes arranged to receive heat mainly'by radiation from theimain combustion-zone. This interposed stratum of relatively low temperature gases functions to reduce substantially the-absorption of heat radiantly transmitted to the furnace wall area so. shielded and to thereby leave a corresponding I greater quantity of heat in the gases leaving the furnaceforsubsequent convection heating of 'thefsuperheater tubes.

In contrast to prior schemes involving intensivedilution .or mixing of the combustion air with inertirecirculated gases, which inherently results in retardation of combustion and .lengthening of theflame, ithe present inventioninvolves introduction of therecirculated gases into-thefurnace inisuch a mannerthat.therecirculated gases'tend to flow along the furnace walls 'iat'velocities which delay, rather than promote, mixing withthe'newly generated high temperature combustion gases. Stable ignition and combusion of the entering fuel results, with no decrease in maximum flame temperatureor decrease in: rate of. flame propagation.

Theamount-of heat radiated by a flowing stream of high temperature gases is not only a function of its temperature but'also of the area of the radiating source. With gas recirculation in accordance withthe invention, thenewly generated combustion gases are crowded by the recirculated gases into a smaller cross-sectional flow area perimeter than without recirculation, over 'amajor portion'of the gas flow path in the furnace. A shorter timeof travel of the high temperature gases 'between'the burners and furnace gas outlet results, due to the increased gas velocity and some decrease in the average length of gas travel in the furnace, but with no decrease in'burner efiiciency, because of the lack of substantial mixing of therecirculated gases and newly generated combustion gases until combustion has been virtually completed.

The'interposition of a thick'stratum of relatively. low temperaturerecirculated gases between the main com- 4 .bustion zone .and .a .large .area .of .furnace wall .tubes coupled with a substantial decrease in the radiating perimeter of the main combustion zone results in a considerable increase in the average temperature of the gases leaving the furnace over the furnace exit gas temperature at the same operating load without recirculation.

Consequently by the use of the present invention,-the superheater tubes will be contacted by a greater mass flow of gases at a higher temperature than without recirculation, and a substantially higher superheated steam temperature will be attained. The rise in superheat temperature is dependent upon the amount of gases'so recirculated. As compared with prior gas recirculation schemes, the present method provides a greater rise in superheat temperature for a given amount of recirculated gases. This method of gas recirculation thus makes it possible to regulate convection superheat temperatures over a wide range of boiler load, without sacrificing either combustion efiiciency or stability.

The present'method of gas recirculation is especially designed and particularly useful with short flame turbulent burners. With-such burners combustion islargely completed and the maximum furnace temperature attained only a few feet from the burner port. The-stream of'high temperature combustion gases generated'rapidly expands-to fill thefurnace and radiate heat to the furnace walls. With gas recirculation this gas expansion is not permitted to'the same extent due to the deflection and crowding action of the recirculated gases which develop a lowtemperature' thick gas stratum between a part of the furnace wall'area and the expanding gases.

"-Theinvention is useful not only in maintaining a uniform or substantially uniform superheat temperature overaa wide range of operating loads, but also for increasingsuperheat temperatures during low loadstarting up periods when it is undesirable to subject turbine parts to steam temperatures below a predetermined minimum value.

'A furtherfeature of the invention is the provision of anautomatically controlled damper in the recirculated gas duct between the control damper thereof and furnace inlet, to permit a slight inflow of cool atmospheric air into the duct and furnace when the control damper is in its closed position. This will eliminate any tendency forhot furnace gases to flow from the furnace through the recirculating duct system if the control damper-is not completely tight in that position.

Theinvention will be described with reference to ap' paratus shown in the accompanying drawings, and 'further features of the invention will appear as the description proceeds.

In the drawings:

Fig. 1 is an elevation partly in section of a steam boiler constructed for operation'in accordance with the invention;

Fig. 2' is anenlarged multiple plane horizontal 'crosssectionalong the line 22 of'Fig. 1;

*Fig. 3 is aniiagrarnmatic view of the furnace of Fig. 1 indicating the gas flow conditions without recirculation 'of furnac'e gases;

:Figp4-is a plan section diagrammatically illustrating the lfurnacegas flow conditions at the level of'th'e line :4--4, Fig. -3;

Fig. 5 is a view similar to Fig. 4 diagrammatically illustrating the gas flow conditions at the level of the line's- 5 of Fig. 3;

"Fig. 6'is'a'view similar to Fig. 3 indicating the furnace gas flowconditions with gas recirculation in accordance with the invention;

Figs.'7 and 8 are views similar to Figs. 4.and SIindieating the-gas flow conditions at the levels-of the lines 7-7 and 8- 8 of Fig. 6; and

Fig. isa-view similar to Fig. l 'showinga different type of burner and amodified arrangement for theintroduction of recirculated gases.

In Figs. 1 and 2 of the drawings the invention is illustrated as embodied in a steam' generating. and superheating unit of known design having a vertically elongated furnace chamber of rectangular cross section, the front and rear walls 9 and 11 of which are delineated by upright steam generating tubes 12 and 14 respectively. The opposite side walls 17 (Fig. 2) have similar wall tubes directly connecting top and bottom headers 13 and 15 respectively. The portions of the front and side walls at the burner level are of refractory covered stud tube construction, as showninFig. 2. All of the wall tubes are connected into the circulation system which includes an upper stearnand water drum 16 and a lower water drum 18, connected by banks 20 and 22 of steam generating tubes. The tubes 12 and 14 discharge into the drum 16 and have headers 82 and 80 respectively at their lower ends, connected in a well known manner to the drum 18. In the path of the gases flowing from the furnace are groups of bent screen tubes 14 forming continuations of the rear wall tubes 14. A convection heated pendent type superheater, formed by two banks of tubes 24 and 26, is positioned in the gas flow path between the screen tubes and generating bank 20, the screen tubes being arranged to screen the superheater tubes from furnace radiation. In this instance the superheater tubes receive heat from the gases by radiant and convection transmission from the stream of heating gases, but the convection transmission is predominant because of the small lateral spacing of the tubes and the relative temperature of the gases. If the superheater were located in a higher gas temperature zone and with a greater lateral spacing, the proportion of heat transmitted by radiation as compared with convection transmission would be increased. The steam flows from the steam space of the drum 16through tubes 28 to the bank of tubes 24 and intermediate header 32 and thence through the bank of tubes 26 to the superheater outlet header 34.

The furnace chamber 10 is fired by two rows of horizontally arranged burners 36 and 38 disposed at different levels, and directing burning fuel and air'in mixing relationship through corresponding burner port's 37'an'd 39 respectively in the front wall 9 into the furnace chamber. The burners are fired by pulverized fuel delivered from pulverizers (not shown) in suspension in primary combustion air through burner supply pipes 40 and 42 respectively. Heater secondary combustion air is supplied under a positive pressure from a windbox 44 enclosing the burner ports 37, 39.

The furnace illustrated is of the water cooled hopper bottom type with the rear furnace wall 11 and the rear sloping hopper wall 48 having the tubes 14 arranged in a tube-to-tube construction. Between the rear wall 48 and the sloping front wall 52 of the hopper 50, there is a throat opening 54 through which the incombustible solid residues of combustion pass into a closed ash pit 57 having opposite walls 56 and 58.

' The pulverized fuel burners 36 and 38 are high capacity burners of the short flame turbulent type. The burners are arranged to discharge pulverized fuel and combustion air in intimate mixing relationship into the furnace through the corresponding front wall burner ports. The mixtures of fuel and air are ignited immediately on entering the furnace and the burning fuel and air mixtures move toward the rear wall of the furnace at a substantial velocity, but the front to back dimension of the furnace is so designed that, even .at maximum high velocity introduction of fuel and air, the combustion products will not impinge to any substantial extent upon the rear furnace wall 11.

While the burning fuel and air mixtures expand greatly on ignition, their velocity of introduction is not quickly dissipated so that the center section of the furnace, i. e. the main combustion zone, may be considered to be filled with an expanding stream of burning fuel, air and products of combustion at a substantial velocity which,

under the influence of the furnace draft, turns upwardly towards the furnace gas exit 45.

With the walls of the furnace defined by rows of generating tubes absorbing heat radiantly transmitted theretofrom the burning fuel and heating gases gen erated thereby, it is necessary to position the burners 36 and 38 at elevations sufliciently below the furnace gas exit so that the gases will be cooled to such a degree that slagging of the screen tubes 14 and superheaterv filled with lower velocity gas eddy currents. The hopper gases however do transmit considerable heat which is absorbed by the steam generating tubes lining the hopper walls.

The heating gas stream leaving the furnace outlet 45 flows across the screen tubes 14 and superheater tube banks 26 and 24 and through the bafiled boiler tube banks 20 and 22. The gases leave the boiler bank 22 at the upper portion thereof and the major portion of the gases pass as indicated by arrows 62 and 64 to a regenerative type air heater from which they pass to draft inducing apparatus, such as an induced draft fan and/or stack (not shown), for discharge to the atmosphere.

A gas recirculation system, operable in accordance with the invention to return a portion of the inert gases leaving the boiler bank 22, is provided by a vertical gas duct 63, recirculating fan 66, and serially connected discharge ducts 68, 69 and 70, the duct 70 extending along and substantially throughout one side of the hopper throat 54 and having discharge flow passages between the spaced lower ends 74 and 76 of the tubes 14 into the throat 54. Duct 63 contains a control and shut-off damper operable from an external arm 92 connected by linkage 94 and an arm 96 to a suitable power operating device 98, as indicated. Duct 68 is also provided with a port 84 in one side thereof between the damper 90 and furnace end and in which a resiliently loaded damper 86 of any suitable type is positioned.

Figs. 3 to 5 of the drawings indicate the general nature of the flow of the furnace gases when the unit is operated in the customary manner at a fractional load with only the lower burners 38 in service. As indicated by the arrows in Fig. 3 and the cross-sectional views of Figs. 4 and 5, the main stream of the high temperature gaseous products of combustion expands rapidly throughout most of the portion of the furnace above the level of the hopper, filling substantially all of the furnace volume except the corners and hopper with high velocity gases moving upwardly to the furnace gas outlet 45. The gas velocities along the furnace walls and in the corners are lowered by the flow retarding effect of the walls. The cross-section occupied by the high velocity gas stream at the burner and furnace outlet levels, is thus approximately as indicated by the broken lines in Figs. 4 and 5 respectively. Eddy currents of hot gases, as indicated by the arrows in Fig. 3, tend to be established within the hopper 50 and cause a transfer of heat to the water tubes lining the hopper walls 48 and 52, as well as the portions of the opposite side walls 17 defining the ends of the hopper 50. The fuel and combustion air supplied to boilers of this type are regulated in a well known manner in accordance with boiler load conditions to provide the desired rate of heat release in the furnace to satisfy the demand for steam. It is well known that convection superheaters of the character disclosed have a rising" curve of superheat temperature as the operating load increases.

In accordance with the present invention, the decrease in superheat temperatures with decreases 'in load can be wholly or partially offset by variable operation of the inert gas recirculation provisions described. At fractional loads either row of fuel burners may be used, although for the lowest operating loads operation of the upper row of burners 36 alone is preferred from the standpoint of super heat control.

With the fuel burners 38 alone in operation and the boiler delivering heating gases to the air heater, the recirculating fan 66 may be operated to withdraw a portion of the gases from the boiler gas outlet, as indicated by the arrow 65, and discharge them through the discharge ducts 68, 69, 70 when the damper 90 .is in an open position. The recirculated inert gases are delivered .by the fan to the furnace hopper throat, entering the throat with only sufficient velocity to insure uniform distribution along the length thereof. This velocity isless than the velocity of the 'fuel and air streams issuing from thefuel burners. The low velocity recirculated gases move upwardly in the diverging hopper and suppress the tendency of eddy currents of freshly developed high temperature combustion gases to form and circulate within the hopper zone. The continuous introduction of the cooler inert gases modifies the character of the flowpath of the stream of high temperature products of combustion generated by the newly'burned fuel, as the gas stream flows towards the furnace gas outlet.

'Figs. 6 to '8 illustrate diagrammatically the general nature of gas flow within the furnace when the unit is operated with recirculation of relatively cool gases in accordance with the invention. As indicated by the broken arrows inFig.-6, therecirculated gases delivered from the recirculating 'fan pass upwardly through the hopper throat and spread out in the upwardly diverging hopper to blanket the hopper walls 48 and 52 and hopper portions of the side walls 17 with a continuously replaced upwardly moving thick stratum of relatively cool gases. The thickness of the recirculated gas layer is difiicult to measure, but in general, a layer from 12-30" in thickness is believed to be desirable. The continuous introduction of recirculated gases inhibits thetendency for hot gas eddy currents to develop along the boundaries of the main gas stream and the mass of .recirculatedgas crowds the stream of freshly developed products of combustion upwardly and inwardly. .The velocity of the recirculated gasesas introduced is sufficiently low, and maintained so by the diverging cross-section .of the hopper, .that .mixing of .freshly developed products of combustion. and the recirculated gases is substantially avoided in the hopper zone of thefurnace. Thereis agradual diifusionof the gas streams along their boundaries, but appreciable mixing of the high and low temperature gas streams apparently does not take place until the gas streamshave passed through a major portion of thegas .flow pathin the furnace.

The recirculated gases are forced out of the hopper zone by the continuous introduction of additional recirculated gases. They pass upwardly alongside the horizontally and upwardly moving high velocity stream of high temperature gases resulting from burner operation and, as indicated in Figs. 7 and 8, tend to embrace a major portion of the periphery (indicated in broken lines in Figs. 7.and 8:) of the streamof high temperature gases. Thestream of high temperature gases thus tends to be crowded into a smaller portion. of the .furnace cross-section than ifnorecirculatiou is provided, as-in Figs. 4 and 5,

because ofthe peripherallyflowing strata of low temperature recirculated gases.

Such crowding of the newly developed high temperature ,products of combustion to a stream of smaller cross-sectional flow area is considered-to have two effects reducing transfer .ofheat therefromto the furnace wall :tubes.

Secondly, l

there will be "a reductionin heat'transfer from the main combustion zone 'to'the surrounding low temperature gas stream and finally to the furnace Walls. As the central hot gas stream originating from the burning fuel is the only source of added heat at a temperature level substantially above that of the heat receiving wall tubes, a considerable reduction in radiant heat transmission to the walls will occur when the higher velocity central stream is of smaller perimeter. A reduction in heat transmitted to the hopper walls 'and to the upright furnace walls results in a greater heat content in and a higher temperature for the gases leavingthe furnace, so that the desired increased convection and gas radiation heating of the superheater tubes is readily effected.

The illustrated "steam boiler installation has a designed maximum steam deliveryrate of 160,000 lbs. of steam per hour at a superheat temperature of the order of 900" F. and a pressure of 660 p. s. i. Tests carried out in the operation of this installation have indicated that at a steam delivery rate of 80,000 lbs. of steam per hr., i. e. of fullload, the superheated steam temperature with only the lower row of burners 38in service and in the customary manner without gas recirculation is only 750 F. Testshave shown that by gas recirculation in accord ance with the invention, the superheat steam temperature can be raised to 900 F. by use of a recirculation rate of approximately 28% of the weight of the gases developed by the combustion of the fuel, when the lower burners alone are in service and at the same operating load.

'When the three upper burners 36 only wereused, for a corresponding operating load, the superheated steam temperature without gas recirculation was 810 F. It was found that a lower rate of gas recirculation than that necessary wtih thelower row of burners alone would readily raise the superheated steam temperature to 900 F.

Operation of the described unit with and without gas recirculation at much lower operating loads than the examples cited above has proven that the method of the invention is effective in attaining the desired superheat temperature at a very low steam delivery rate, with either row of burners in operation. Under even these low load conditions the flame is stable and flame temperature determinations'by customary measurement methods show no significant change in flame temperatures due to the use of recirculated gases.

The resiliently loaded damper 86 arranged in port 84 in-the side wall of the duct 68 at a position between the damper 90 and the furnace discharge openings from the discharge duct 70 is advantageous in maintaining satisfactory conditions within the gas recirculating system when it is not in use, as for example, at high load rates of operation when the desired superheat temperature is attainable without gas recirculation. Under such conditions when fan 66 is not operated and damper is in a closed position .for the purpose of avoiding any reverse how of gases through ducts 70, 69, 68, and 63,.the absolute pressure on the fan side of damper 90 will correspond to that at the position where duct 63 is connected to the gas outlet from boiler bank 22. The resiliently loaded damper 86 is constructed so that when the pressure in the duct is above a predetermined pressure relative to the pressure in the throat of the furnace hopper, it will'be held in a closed position. When the pressure at the damper 86 corresponds to or is below that of the predetermined pressure, atmospheric pressure will overcome the damper loading, opening the damper so that atmos- 'phericair will be drawn through the port 84 into duct 68. This automatic introduction of relatively low temperature atmospheric air will cause a flow of cool air rather than gases from the furnace throat back through duct 68, ten 'andzduct-63in case the damper 90 should not be gas .tight in its closed position.

Variation in :therate of delivery of recirculated gases is accomplished byadjustment of the position of 'damper :90. This may he done .manually by the operator in accordance with indications of superheated steam outlet 9 temperature; However, the damper operating device 98 is adapted for connection to automatic steam temperature measurement apparatus (not shown), so that the rate of gas recirculation may automatically be regulated in accordance with steam superheat temperature. As the superheat temperature tends to fall with decrease in load,

the damper would thus be automatically adjusted to perinit more gas to be recirculated, and vice versa.

While the increase of steam superheat temperature by means of gas recirculation to the furnace is of great value for superheat temperature control in connection with the normal operating load range of the boiler unit, the invention is also of particular advantage in starting up boiler units where it is desired to fire the unit and deliver a relatively low rate of steam at a predetermined minimum temperature. Inasmuch as the introduction of .recirculated gases in the manner of the invention does not aflfect burner operation, it is possible to use the method disclosed at a very low delivery rate from the unit. A low steam delivery rate is all that is usually required to turn over. and gradually bring up to speed the prime mover served by the boiler. When a non-operating prime mover, as, for example, a steam turbine, has been retained at high etmperature, it is undesirable to introduce steam of low superheat to bring the turbine up to speed. The use of the invention permits the delivery of steam at a relatively high temperature even though the delivery rate may be as low as to 20% of the full load steam delivery rate of the unit.

The Fig. 9 modification presents an alternate arrangement whereby recirculated gases can be introduced in an effective relationship to heat absorbing hopper wall surfaces for superheat regulation in accordance with the invention. The steam boiler and furnace are of the same general type as that shown by Fig. 1, having a completely Water cooled furnace with a hopper type furnace bottom having water tubes lining the inclined hopper walls. Two rows of burners 136, 138 of a turbulent cross-tube type are arranged to discharge fuel and combustion air rearwardly in the furnace. The burners are positioned in the vertical front wall at a position spaced above the upper edge of the front inclined hopper wall 152. The tubes 112 of the hopper front wall are spaced apart so that the intertube spaces opposite the recirculating gas duct 170 provide introduction ports for recirculated gases which are deliveredsubject to regulation by damper 190 by the recirculating fan 166. A duct 163 connects the' boiler gas outlet-to the inlet of the fan 166, while the control damper 190 and atmospheric damper 186 are arranged between the fan and the ports opening to the furnace.

The free flow areas of the gas introduction ports as determined by their length and the width of the intertube spaces are such that the gas delivered through the ports enters the furnace at relatively low velocity in nonmixing relationship, as regards relative velocities, with the gases developed by direct fuel combustion. The recirculated gases so introduced will spread through the hopper maintaining strata of relatively cool gases between the main combustion zone and the hopper walls, .while the gases from thehopper zone as heated up will pass upward and through the furnace in a mannergenerally similar to the gas flow indicated in Figs. 6-8.

While the invention has been illustrated as used with pulverized fuel burners, it is also adapted for use with other fluid fuel burners, such as oil or gas burners.

The disclosed introduction of a stream or streams of low temperature inert recirculated gas into the furnace in a manner so as to avoid mixing of the streams with either the secondary combustion air or with the mixture of fuel and air delivered through the burner port, is particularly important from the standpoint of efficient and stable burner performance. Dilution of the combustion airwith inert recirculated gases and thereby retardation of combustion and lengthening of the flame is avoided. Retardation of combustion and lengthening of flame in a 10 furnace normally result in a'lower combustion efficiency due' to a higher unconsumed carbon loss, particularly in the fly ash. With the present method of introduction of recirculated gases, no modification or adjustment of the fuel burners is necessary whether or not gas recirculation is in use.

While in accordance with the provisions of the statutes I have illustrated and described herein the best forms of the'inv'ention now known to me, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by my claims, and that certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

I claim:

1. A vapor generating and superheating unit comprising walls arranged to define a furnace chamber having a heating gas outlet, fuel burning means for introducing fluid fuel and combustion supporting air into said furnace chamber at a point remote from said heating gas outlet in a gas flow sense and burning the fuel in suspension therein, vapor generating tubes lining an upwardly extending boundary surface of said furnace chamber spaced from the point of fuel and combustion air entry and arranged to receive heat mainly by radiation from the burning fuel stream, a convection heating section including vapor superheating tubes arranged to receive heat from heating gases leaving said heating gas outlet, means for withdrawing relatively low temperature heating gases from said convection section after loss of heat therefrom to the vapor superheating tubes, means having a recirculated gas'o'utlet spaced from the point of fuel and com bustion air entry and disposed outwardly of the main combustion zone for introducing the withdrawn gases separately from the introduction of combustion supporting air into said furnace chamber at a location at least as far from the heating gas outlet with respect to the gas flow path as the point of fuel and combustion air entry and alongsaid furnace chamber boundary surface in a layer distributed substantially throughout the transverse extent of said boundary surface vapor generating tubes so as to provide and maintain a moving layer of low temperature gas between said furnace chamber boundary surface vapor generating tubes and the burning fuel stream in the main combustion zone, said boundary surface being so arranged relative to said gas introduction means that the gravitational effect on said gas layer tends to maintain said gas. layer in contact with said boundary surface.

2. A vapor generating and supcrheating unit comprising walls arranged to define avertically elongated furnace chamber having a heating gas outlet adjacent one end thereof, fuel burning means introducing fluid fuel and combustion supporting air into said furnace chamber at a point adjacent the opposite end of said furnace chamber and-burning thefuel in suspension therein, vapor generating tubes lining a boundary surface of said furnace chamber spaced from the point of fuel and combustion air entry and arranged to receive heat mainly by radiation from the burning fuel stream, a convection heating section including vapor superheating tubes arranged to receive heat from heating gases leaving said heating gas outlet, means for withdrawing relatively low temperature heating gases from said convection section after loss of vheat therefrom to the vapor superheating tubes, means having a recirculated, gas outlet spaced and substantially separated from the point of fuel and combustion air entry and disposed outwardly of the main combustion zone for introducing the withdrawn gases into said furnace chamber ata location at least as far from the heating gas-outlet ,with respect to the gas flow path as the point of fuel and combustion air entry and along said furnace chamber boundary surface in a layer distributed substantially throughout the transverse extent of said boundary surface vapor generating tubes so as to provide and maintain a moving layer of 'low temperature gas between 'said'fun nace chamber .boundary surface vapor generating tubes and theburning fuel stream in the main combustion 'zone, said boundary surface'being so arranged relative to said gas introduction means that the gravitational effect on said gas layer tends to maintain said gas layer in contact with said boundary surface, the introduction of the with drawn gases taking place separately from the introduction of combustion supporting air.

'3. Avaporgenerating andsuperheating unit comprising walls arranged to define a vertically elongated furnace chamber of rectangular'horizontal cross-section having a heating gas outlet adjacent .its upper end, fuel burning means for introducing fluid fuel and combustion supporting air into the furnace chamber at a level intermediate the height of said furnace chamber and burning the fuel in suspension therein with an upward how of the gaseous products of combustion from said level to said heating gas outlet, means defining a bottom for said furnace chamber belowthelevel of said fuel burning means, said lastnamed means .including vapor generating tubes extending along 'saidfurnace chamber bottom and arranged to receive heat mainly by radiation from the burning fuel, a convection heating section including vapor superheating tubes arranged to receiveheat from heating gases leaving said heating gas outlet, and means for withdrawing heating gases from said convection section after loss of heat therefrom 'tothe vapor superheating tubes and introducing't'ne withdrawn gases along said furnace chamberbottom so as to provide a layer of withdrawn gases between the burning fuel and the vapor generating .tubes extending along said furnace chamber bottom, said last named means'including a recirculated gas outlet communicating with the furnace chamberat a position so substantially spaced from the position of burning fueland air entry'as to prevent the recirculated gas introduction from interfering 'with ignition stabilization, the recirculated gas introduction being substantially separated from the introduction of combustion supporting air and the recirculated gas outlet being disposed substantially outside the main combustion'zone.

4. A vapor generating and superheating unit comprising walls arranged to'define a vertically elongated furnace chamber of rectangular horizontal cross-section having a heating gas outlet adjacent its upper end, fuel burning means for introducing a fluid fuel .and combustion air into said furnace chamber at a level intermediate the height of said furnace chamber and burning the fuel in suspension therein with an upward flow of the gaseous products of combustion from said levelto said heating gas outlet, walls defining a hopper bottom for said furnace chamber below thelevel of said fuel burning means and terminating at its lower end in an elongated throat, vapor generating tubes alongawall of. said hopper and arranged to receive'heat mainly by radiation from the burning fuel, a convection heating section'includingvapor superheating tubes arranged to receive heat from heating gases leaving said heating gas outlet, and-means for withdrawing heating gases from said convection section after the heating gases have passed over at least-amajorportion of the 'vapor superheating tubes and introducing the/withdrawn gases into said'hopper so as to fiow along the vapor generatingtubes in said hopper wall.

5. A vapor generating and superheating unit-comprising walls arranged to define a vertically elongated furnace chamber of rectangular horizontal cross-section having 'a "heating gas outlet adjacent its upper end,fuel'burning means for introducing a fluid fuel "and combustion air through 'one of the vertical walls-of'theiurnace chamber at a level intermediate theheight of said furnace'chamber and burning the fuel in suspension therein with an' upward "flow of thegaseous products 'of'combustionfrom'said'level to-said'heating gas outlet walls defining a'hopperbottom for said furnace chamber disposed *below the level of 7 saidi'uel'burning means and havingatdtslower part a throat 'extending'transversely to the direction of 'fuel entry fora major portion of the width of said furnace chamber, vapor generating tubes extending along a wall of said hopper and arranged to receive'heat mainly by radiation from'the burning fuel, a convection heating section including vapor superheating tubes arranged to receivce heat from heating gases leaving said heating gas outlet, and means for withdrawing heating gases'from said iconvection section after the heating gaseshave passed over at least a'major portion of the vapor superheating'tubes and introducing the withdrawn gases upwardly through the throat of said hopper substantially throughout the lengththereof.

6. A'vapor generating and superheating unit comprising walls arranged to define a vertically elongated:furnace chamber of rectangular horizontal cross-section having a heating gas outlet adjacent its upper end,ffue1'burning means for introducing a fiuid fuel and combustion air through one of the vertical walls of'the furnace chamber at a level intermediate the height of said ifurnacechamher and burning the fuel in suspension therein with an upward flow of the gaseous products of combustion from said'level to said heating gas outlet, a pairof oppositely inclined walls defining a hopper bottom'for said furnace chamber symmetrically arranged below the level of said fuel burning means and terminating at its'lower en'd in.a throat extending transversely to the direction of fuel entry substantially throughout the width of said furnace.chamber, vapor generating tubes extending alongtsaid'furnace walls and said hopper inclined walls and arranged to receive heat mainly byradiation from the burning fuel, a convection heating section including 'vapor superheating tubes'arranged to receive heat from heating igasesleaving said heating gas outlet, and means for withdrawing'heating gases from said convection section after the' heating gases have passed over at least a major portion of the vapor superheating tubes and introducing the withdrawn gases upwardly through the throat of said hopper substantially throughout the length thereof.

7. The method of operating a vapor generating and superheating unit having walls defining ,a vertically elongated furnace chamber having a'heating gas outlet adjacent its upper end, vapor generating tubes' lining aboundary surface of said furnace chamber'and arranged to receive heat mainly by radiation, and a vapor superheater in the path of heating gas fiow from said'heatinggas outlet and arranged to receive heat mainly by convection; the method comprising the introducing of'fluid'fuel and combustion supporting air into said furnace chamber in :a generally horizontal direction at a'levelintermediate the height of siad furnace chamber and burning the fuel in suspension therein while the stream of burning fuel, combustion air and heating gas generated flows upwardly in said furnace chamber towards said heating gas outlet through a main gas flow path occupyingless than the full cross-sectional area of said furnace .chamber during operation over afractional load range; and raising the'vapor superheat temperature at such'fractional loads by withdrawing relatively low temperature heating gas after loss of heat therefrom in the superheating and introducingrthe withdrawn heating gas into the lower portion of said furnace chamber below the level ofsaidfluid'fuel and combustion air introduction and below said main heating gas flow path and in a direction and ata discharge velocity causing the introduced gas to ,flow initially betweensaid main heating gas flow path and the vapor generatingtubes lining said furnace chamber boundary surface and thence towards said heating gas outlet, the recirculated gases being introduced into the furnace separatelyvfromihe combustion supporting air at,a,position outside :of the main combustion zone.

8. .The method of operating a vapor generating aud superheating unit'having walls .definingta vertically elongated furnace chamber of rectangular horizontal .crosssectionhaving'a heating gas outlet adjacent its upper end,

13 vapor generating tubes lining a wall of saidfurnace chamber and arranged to receive heat mainly by radiation, and a vapor superheater in the path of heating gasrflow from said heating gas outlet and arranged to receive heat mainly by convection; the method comprisingthe introducing of'fluid fuel and combustion supporting air into said furnace chamber in a generally horizontal direction at a level intermediate the height of said furnace chamber and burning the fuel in suspension therein while the stream of burning fuel, combustion air and heating gas generated .flows upwardly in said furnace chamber towards said heating gas outlet through a main "gasflow path occupying less than. the full cross-sectional area of said furnace chamber during operation overza fractional load range; and raising the vapor superheat temperature at such fractional loads by withdrawing relatively low temperature heating gas after loss of heat therefrom in the-superheating and introducing the withdrawn heating gas into the lower portion of said furnace chamber below the level of said fluid fuel and combustion air introduction and said main heating gas flow path and in a direction and at a discharge velocity causing the introduced gas 'to flow initially between said main heating gas flow path and the vapor generating tubes lining said furnace chamber wall andv thence towards said heating gas outlet, the introduction of recirculated gasesbeing separate from the introduction of combustion supporting air/ I 9. In a method of controlling superheat over a wide range of vapor generation, generating vapor at different rates bylthe radiant transmission of heat from' a combustion zone to confined streams of avaporizable liquid, superheating the generated vapor by convection heat transfer thereto from gases from the combustion zone,

maintaining combustion in said zone by projecting a stream of fluid fuel and combustion air'into said zone for flow of thee'ombustion gases in a main flow pathto the superheating zone, increasing the temperature of. the superheated vapor over a fractional load range. by withdrawing relatively low temperature'heating gases after loss of heat therefrom in said superheating and introducing the withdrawn gases into contact with the unrecirculated gases of said combustion zone at a position below the level of the fluid fuel and combustion air introduction and below the level of the main heating gas flow path a and at a discharge velocity causing the introduced gases to flow initially as a stratum between said main heating gas flow pathand the confined streams of liquid and simultaneously causing the main gas flow path of the unrecirculated combustion gases to be reduced in cross section as the rate of vapor generation decreases.

10. In a method of controlling superheat over a wide range of vapor generation, generating vapor at different rates by the radiant transmission of heat from a combus tion zone to confined streams of vaporizable fluid, superheating the generated vapor by convection heat transfer thereto from the gases from the combustion zone, maintaining combustion in said zone by projecting a stream .of fluid fuel and combustion air into said zone for the flow of combustion gases in a main gas flow path to the superheating zone, maintaining the superheat temperature at a predetermined value over a wide range of fractional loads by withdrawing relatively low temperature heating 7 gases after loss of heattherefr'om in said superheating and introducing the withdrawn gases into contact with the combustion gases at a position below the level of the fluid fuel and combustion air introduction and below the level of the main gas flow path where the force of gravity tends to maintain the introduced gases as a stratum interposed relative to the main gas flow path and said confined streams of vaporizable fluid, said introduction of gases taking place at such a velocity lower than the velocity of the entering fuel stream that the maintenance of said stratum is promoted, said introduction of withdrawn gases being increased as the rate of vapor generation decreases and thereby causing a cross-sectional reduction of said main gas flow path.

11. In a method of controlling superheat over -a wide range of vapor generation, generatingvapor at different rates by the radiant transmission of heat from a combustion zone to confined streams of vaporizable fluid, superheating, the generated vapor by convection heat transfer thereto from the gases from the combustion zone, maintaining combustion in said zone by horizontally projecting a stream offluid fuel and combustion air into said zone for the upward flow of combustion gases in a main gas flow path to the superheating zone, maintaining the superheat temperature at a predetermined value over a wide range of fractional loads by withdrawing relatively low temperature heating gases after loss of heat therefrom in said superheating and introducing the withdrawn gases for upward flowinto contact with the combustion gases of the main gas flow path at such a position below the level of fuel entry that the force of gravity tends to maintain the introduced gases as a stratum interposed relative to the-main gas flow path and said confined streams of vaporizable fluid, said introduction of gases taking place at a level below the main gas flow path and at such a velocity lowerthan the velocity of the entering fuel stream that the maintenance of said stratum is promoted, said introductionof withdrawn gases being increased as the rate of vapor generation decreases and thereby causing a cross-sectional reduction of said main gas flow path.

112. The method of operating a vapor generating and superheating unit having walls defining a, vertically elongated furnace chamber having a heating gas outlet adjacent one end thereof, vapor generating tubes lining wall of said furnace chamber and arranged to receive heat mainly by radiation, and a vapor superheater in the path of heating" gas flow from said'heating gas outlet and arranged to receive heat mainly by convection; the method comprising introducing fluid fuel'and combustion air into said furnacechamber and burning the fuel in suspension therein while the stream of burning fuel, combustion air and heating gas generatedflows towards said heating gas outlet'through a main gas flow path occupying less than the full cross-sectional area of said furnace chamber during operation over a fractional load range; and raising the vapor superheat temperature at such loads by withdrawing relatively low temperature heating gas from said unit after loss of heat therefrom to said vapor superheater and providing and maintaining a moving layer of low temperature gas initially between said main heating gas flow path and the vapor generating'tubes lining the furnace chamber wall by introducing the withdrawn heating gas into said furnace chamber in a mixing retarding manner at a location spaced and separate from the fuel and combustion air introduction and disposed outwardly of the main combustion zone and in a layer distributed substantially throughout the transverse extent of said vapor generating wall tubes, said gas layer being so directed that the gravitational effect thereon tends to minimize mixing of the gas layer with the unrecirculated products of combustion and to maintain the gas layer in contact with the vapor generating tubes.

13. In a steam generating and superheating unit comprising walls defining a vertically elongated furnace chamber having a heating gas outlet at the upper end thereof; steam generating tubes lining a vertical wall of said furnace chamber; means at a level intermediate the vertical extent of said steam generating tubes for introducing and burning a stream of fuel and air in suspension in said furnace chamber; and a bank of steam superheating tubes arranged to be heated mainly by convection by heating gases flowing through said heating gas outlet; the method of controlling the convection steam superheating effect which comprises withdrawing heating gases from a position in the heating gas flow path beyond said steam superheating tubes and separately introducing substantially the entire volume of withdrawn gases into said furnace chamber at a position more remote in the gas flow path from the heating gas outlet than the position of fuel introduction and sufiiciently below the level of introduction of the fuel and air-stream, belowthe lowest plane of'burning fuel mass, "at a relatively "low velocity substantially less thanthe velocity of fuel-introduction and at an angle :to minimize interference with the ignition of the fuel as introduced and to maintain-a body of recirculated gas-be tween the-main-combustion zone and steam generating wall tube portions below the lowestplane of burning'fuel mass.

14. In a steam generating and superheating unit comprisingwalls defining a vertically elongated furnace chamber having a heating gas outlet at the upper end thereof and a hopper atthe lower end thereof; steam generating tubes lining a vertical wall of said furnace chamber and hopper; means 'at a'levelintermediate thevertical extent of said steam generating'tubes for introducing and burn ing a stream of "fuel and air in'suspension in said furnace chamber; and abank of steam superheating tubes arranged to be heated mainlyby convection by heating gases flowing through said'heatinggas outlet; the method of controlling the convection steam superheating effect which comprises withdrawing heating-gases from a position in the heating gas flow path beyond said steam superheating tubes and separately introducing substantially the entire volume of withdrawngases-into said furnace chamber ataposition more remote in the'gas flow path from the heating gas outlet than the position of fuel introduction'and su'fficiently belowthe-level of introduction of the fuel and-air stream, below the lowest plane of burning fuel-mass, -at a relatively low velocity substantially less than-the velocity of fuelintroduction and at an angle to-minimize interferencewith-theignition of the fuel as introduced and to-maintain a body of recirculated gas between the maintcombustion'zone and steam generating tube portionslining said 1 01 12 l and below thelowest level of burning fuel.

15. vA steam generating and ,superheating nnitcomprising walls defininga -vertically;elongat,ed furnace chamber having a heating gas outlet at the upperend thereof :and

ahopperatthedower end-thereof; steam generating tubes lining a-vertical wall of said furnace chamber and hopper; means at a level intermediate the vertical extentof said steam generating tubes for introducing and burning a-stream of fuel andsairin suspension in said furnace chamber; a bank of steam superheating tubes arranged to 'be-heated mainly ,by convection by the gases flowing through-said outlet; and means for controlling .thevconvection superheating effect including a gas recirculating fan for Withdrawing heating gases from a position in said heating :gas flow path beyond said steam superheating tubes :and separately-introducing substantially the entire volume of withdrawn gases into said furnace chamber at a position more .remote in the heating gas flow path from'tthe "heating gas outlet than the position of fuel introductionand sufficiently below the level of'the introduction.of the-fuel and air stream, below the lowest plane of :burning fuel mass, at a relatively low velocity -'substantiallyless thanrthe velocity of fuel introduction, and at -.an angle, :to :minimize interference with the ignition of :the duel asintroducedand to maintain a body of recirculated ;gas between ,the main combustion zone and steam .generating tube portions lining said hopper .wall

and :belowtthe lowest level of burning fuel.

References Cited in the file of this patent I UNITED STATES PATENTS 

1. A VAPOR GENERATING AND SUPERHEATING UNIT COMPRISING WALLS ARRANGED TO DEFINE A FURNACE CHAMBER HAVING A HEATING GAS OUTLET, FUEL BURNING MEANS FOR INTRODUCING FLUID FUEL AND COMBUSTION SUPPORTING AIR INTO SAID FURNANCE CHAMBER AT A POINT REMOTE FROM SAID HEATING GAS OUTLET IN A GAS FLOW SENSE AND BURNING THE FUEL IN SUSPENSION THEREIN, VAPOR GENERATING TUBES LINING AN UPWARDLY EXTENDING BOUNDARY SURFACE OF SAID FURNACE CHAMBER SPACED FROM THE POINT OF FUEL AND COMBUSTION AIR ENTRY AND ARRANGED TO RECEIVE HEAT MAINLY BY RADIATION FROM THE BURNING FUEL STREAM, A CONVECTION HEATING SECTION INCLUDING VAPOR SUPERHEATING TUBES ARRANGED TO RECEIVE HEAT FROM HEATING GASES LEAVING SAID HEATING GAS OUTLET, MEANS FOR WITHDRAWING RELATIVELY LOW TEMPERATURE HEATING GASES FROM SAID CONVECTION SECTION AFTER LOSS OF HEAT THEREFROM TO THE VAPOR SUPERHEATING TUBES, MEANS HAVING A RECIRCULATED GAS OUTLET SPACED FROM THE POINT OF FUEL AND COMBUSTION AIR ENTRY AND DISPOSED OUTWARDLY OF THE MAIN COMBUSTION ZONE FOR INTRODUCING THE WITHDRAWN GASES SEPARATELY FROM THE INTRODUCTION OF COMBUSTION SUPPORTING AIR INTO SAID FURNACE CHAMBER AT A LOCATION AT LEAST AS FAR FROM THE HEATING GAS OUTLET WITH RESPECT TO THE GAS FLOW PATH AS THE POINT OF FUEL AND COMBUSTION AIR ENTRY AND ALONG SAID FURNACE CHAMBER BOUNDARY SURFACE IN A LAYER DISTRIBUTED SUBSTANTIALLY THROUGHOUT THE TRANSVERSE EXTENT OF SAID BOUNDARY SURFACE VAPOR GENERATING TUBES SO AS TO PROVIDE AND MAINTAIN A MOVING LAYER OF LOW TEMPERATURE GAS BETWEEN SAID FURNACE CHAMBER BOUNDARY SURFACE VAPOR GENERATING TUBES AND THE BURNING FUEL 